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The earthquake (ML5.2) which occurred at Chushan town, Nantou on Nov.11, 2017 resulted many aftershocks around the hypocenter of the mainshock. The coulomb stress changes (∆CFSs) by the mainshock demonstrated by most of the aftershocks located within the portions with ∆CFSs greater than 0.1 bar, revealing the aftershocks may be induced by the mainshock. After investigating the ∆CFSs at different depths, the results showed most of ∆CFSs with positive values under depth 15km were transferred to the southeast and southwest regions. About ten days later, there were many earthquakes (ML5.2~3) with depth below 15km clustered exactly in the southeast region. The southwest region near the Meishan fault, caused large earthquake with ML7.0 before 110 years ago. This means the Meishan fault may be triggered by the successive earthquakes near the fault. Based on the seismic spectrum intensities, the seismic hazards by the potential Meishan earthquake are estimated. The maximum seismic intensities will occur at Chushan township and the segment of freeway 3 connecting Nantou city and Chushan township may be damaged seriously. The landslides may happen again at the junctions of Nantou and Yunlin counties, where great landslides happened when Chi-Chi earthquake (ML7.3) occurred in 1999.

The 2019–2020 Southwest Puerto Rico earthquake sequence ruptured multiple faults with several moderate magnitude earthquakes. Here, we investigate the seismotectonics of this fault system using high-precision hypocenter relocation and inversion of the near-field strong motions of the five largest events in the sequence (5.6≤Mw≤6.4) for kinematic rupture models. The Mw 6.4 mainshock occurred on a northeast-striking, southeast-dipping normal fault. The rupture nucleated offshore ∼15 km southeast of Indios at the depth of 8.6 km and extended southwest–northeast and up-dip with an average speed of 1.55 km/s, reaching the seafloor and shoreline after about 8 s. The 6 January 2020 (10:32:23) Mw 5.7 and the 7 January 2020 (11:18:46) Mw 5.8 events occurred on two east–southeast-striking, near-vertical, left-lateral strike-slip faults. However, the 7 January 2020 (08:34:05) Mw 5.6 normal-faulting aftershock, which occurred only 10 min after the Mw 6.4 normal-faulting mainshock, ruptured on a fault with almost the same strike as the mainshock but situated ∼8 km farther east, forming a set of parallel faults in the fault system. On 11 January 2020, an Mw 6.0 earthquake occurred on a north–northeast-striking, westing-dipping fault, orthogonal to the faults hosting the strike-slip earthquakes. We apply template matching for the detection of missed, small-magnitude earthquakes to study the spatial evolution of the main part of the sequence. Using the template-matching results along with Global Positioning System analysis, we image the temporal evolution of a foreshock sequence (Caja swarm). We propose that the swarm and the main sequence were a response to a tectonic transient that most affected the whole Puerto Rico Island.

The 30 October 2020 Samos earthquake (Mw 7.0) ruptured an east–west striking, north dipping normal fault located offshore the northern coast of Samos Island, previously inferred from the bathymetry and regional tectonics. This fault, reported in the fault-databases as the North Samos and/or Kaystrios Fault, ruptured with almost pure dip-slip motion, in a region where both active extension and strike-slip deformation coexist. Historical information for the area confirms that similar ~ Mw7 events had also occurred in the broader Samos area, though none of the recent (last ~ 300 years) mainshocks appears to have ruptured the same fault. The spatial and temporal distribution of relocated aftershocks indicates triggering of nearby strike-slip and normal fault segments, situated in the areas where static stress has increased due to the mainshock generation. The relocated aftershocks and the slip model indicate that the sequence ruptured the upper crust (mainly the depth range 3–15 km). The top of the rupture plane nearly reached the sea bottom, located at a depth of

Numerous shallow earthquakes, including 24th August Amatrice, 26th October Visso, and 30th October Norcia earthquakes, ruptured the segments of Mount Vettore-Gorzano fault system in the central Apennines (Italy) in 2016. In order to investigate the stress perturbation and triggering patterns among the earthquake sequences, we introduce a more realistic nonplanar coseismic fault geometry model, which improve the rupture model by assimilating relocated aftershocks and the GPS observations. We adopt the seismic slip inversion program of the steepest descent method (SDM) to create the detailed coseismic rupture models and optimize Coulomb Failure Stress model by varying the coefficient of friction and received fault parameters. The results indicate that the nonplanar fault geometry model is more reflective of the deep slip of the coseismic rupture than planar model. As evidenced by the coseismic Coulomb stress changes caused by the three mainshocks at different depth slices, the stress loading mainly distributes on the active fault zones and the stress changes can well explain the spatial distribution of aftershocks. The first large Amatrice mainshock accelerates the occurrence of the Mw 5.9 Visso and Mw 6.6 Norcia earthquakes, with the positive stress changes at the hypocenter exceeding the stress triggering threshold (0.010×106 Pa) and up to 0.015×106 and 0.257×106 Pa, respectively. Furthermore, the Mw 5.9 Visso earthquake as well encourages the occurrence of the Mw 6.6 Norcia event with the increased stress changes of 0.052×106 Pa on the hypocenter. It is concluded that the stress transfer and accumulation play crucial roles on the linkage triggering mechanism among the mainshock-mainshock and mainshock-aftershocks. Noteworthily, the cumulative stress changes on the southwest segment of the Norcia Fault (NF), the southeast parts of the Montereale Fault System (MFS) and Mount Gorzano Fault (MGF) of the main regions are up to (1.5∼3.5) ×106 Pa. The cumulative stress changes have not been released sufficiently by aftershocks, which may increase the seismic hazard in those regions.

This study comprehensively investigates the source mechanisms associated with the mainshock and aftershocks of the Mw = 6.3 Yogyakarta earthquake which occurred on May 27, 2006. The process involved using moment tensor inversion to determine the fault plane parameters and joint inversion which were further applied to understand the spatial and temporal slip distributions during the earthquake. Moreover, coseismal slip distribution was overlaid with the relocated aftershock distribution to determine the stress field variations around the tectonic area. Meanwhile, the moment tensor inversion made use of near-field data and its Green’s function was calculated using the extended reflectivity method while the joint inversion used near-field and teleseismic body wave data which were computed using the Kikuchi and Kanamori methods. These data were filtered through a trial-and-error method using a bandpass filter with frequency pairs and velocity models from several previous studies. Furthermore, the Akaike Bayesian Information Criterion (ABIC) method was applied to obtain more stable inversion results and different fault types were discovered. Strike–slip and dip-normal were recorded for the mainshock and similar types were recorded for the 8th aftershock while the 9th and 16th June were strike slips. However, the fault slip distribution from the joint inversion showed two asperities. The maximum slip was 0.78 m with the first asperity observed at 10 km south/north of the mainshock hypocenter. The source parameters discovered include total seismic moment M0 = 0.4311E + 19 (Nm) or Mw = 6.4 with a depth of 12 km and a duration of 28 s. The slip distribution overlaid with the aftershock distribution showed the tendency of the aftershock to occur around the asperities zone while a normal oblique focus mechanism was found using the joint inversion.

As basic data for seismic activity analysis, hypocenter catalogs need to be accurate, complete, homogeneous, and consistent. Therefore, clarifying systematic errors in catalogs is an important discipline of seismicity research. This review presents a systematic model-based methodology to reveal various biases and the results of the following analyses. (1) It is critical to examine whether there is a non-stationary systematic estimation bias in earthquake magnitudes in a hypocenter catalog. (2) Most earthquake catalogs covering long periods are not homogeneous in space, time, and magnitude ranges. Earthquake network structures and seismometers change over time, and therefore, earthquake detection rates change over time and space. Even in the short term, many aftershocks immediately following large earthquakes are often undetected, and the detection rate varies, depending on the elapsed time and location. By establishing a detection rate function, the actual seismic activity and the spatiotemporal heterogeneity of catalogs can be discriminated. (3) Near real-time correction of source locations, far from the seismic observation network, can be implemented based on better determined source location comparisons of other catalogs using the same identified earthquakes. The bias functions were estimated using an empirical Bayes method. I provide examples showing different conclusions about the changes in seismicity from different earthquake catalogs. Through these analyses, I also present actual examples of successful modifications as well as various misleading conclusions about changes in seismic activity. In particular, there is a human made magnitude shift problem included in the global catalog of large earthquakes in the late nineteenth and early twentieth centuries.

We used the most complete instrumental data supplied by the global network, a modern velocity model (ak135), and a modified method of generalized beamforming to relocate the Crimean earthquakes of June 26 and September 11, 1927, and for the larger (magnitudes above 4.5) aftershocks that occurred during the year 1927. For the relocation, we collected all available seismic bulletins from the global network using data from the ISC-GEM project (International Seismological Centre-Global Earthquake Model), EuroSeismos project, the ISS (International Seismological Summary) Bulletin, the archive of the Geophysical Survey of the Russian Academy of Sciences bulletins, and the archive of the Russian State Library. There are many onsets of P and S phases as recorded by numerous seismic stations for the two Crimean earthquakes and six aftershocks; the network geometry is favorable for reliable determination of hypocenter parameters. We believe that our results will be a contribution to the study of aftershocks following the 1927 Crimean earthquakes, and this will make it possible to learn new things about the space-time evolution of the seismic process during the active period from 1927 to 1931.

The longitude, latitude and depth of the hypocenter in 3-D space and the date and time of rupture makes an earthquake a “point” in a spatio-temporal point pattern, observed over a region and months, years or decades. The magnitude of earthquakes marks the point pattern, as would hypocenter depth do if only the longitude and latitude of epicenters were used for location in 2-D space. Stochastic declustering, based on a space–time ETAS model (ETAS: epidemic-type aftershock sequence), is a procedure that can be applied in the preliminary stage of an earthquake catalog data analysis. Stochastic declustering procedures have underlying assumptions, such as the time independence of the background intensity function whether the spatial framework is 2-D or 3-D, and a separate treatment of hypocenter depth from longitude and latitude when the spatial framework is 3-D. Cyclical processes in the Earth, including tides and seasonal surface water loads, can introduce periodic behavior in earthquake occurrence and related variables. The effects of ETAS-based 2-D and 3-D declustering on the outcome of periodicity analyses performed from the resulting earthquake data catalogs are studied. The research objectives and statistical challenges include the detection of periodicities for hypocenter depth in addition to monthly earthquake number, and the risk of missing observations for hypocenter depth when the monthly earthquake number after declustering is zero. A version of the method of multi-frequential periodogram analysis (MFPA) that allows for missing observations in the input temporal series is presented in detail, and applied to hypocenter depth (monthly mean and median) for central and northern California from January 2006 to December 2014. The results obtained for the 2-D and 3-D declustered catalogs are compared with those for the original catalog for this region. A semiannual periodicity in hypocenter depth is detected for the original and 2-D declustered catalogs, and fitted with the goal of relating it to periodicities found in the time series of monthly earthquake numbers. Using these results for central and northern California earthquakes, some of the assumptions on the intensity function of the spatio-temporal point process in stochastic declustering are discussed and future research perspectives are proposed.

From 26 April to 8 May 2020, an earthquake sequence occurred in the Haenam region in southwestern Korea. The main shock with a moment magnitude of 3.2 occurred on 3 May. It was preceded by 38 foreshocks that occurred over a week, with magnitudes ranging from 0.8 to 2.7, and was followed for another week by 35 aftershocks with magnitudes ranging from 0.5 to 2.4. The P-wave first-motion indicates that the focal mechanism of the mainshock was a strike of 193° or 102°, a dip of 83° or 78°, and a rake of –168° or –7°. The stress orientation of the focal mechanism is consistent with the regional tectonic stress field. Spectral analysis of the mainshock indicates that it occurred on a fault with a radius of 216 ± 4.58 m and a stress drop of 31.4 ± 2.35 bar. The hypocenters of the sequence, determined using the double-difference method, were shown to cluster along a trend striking 100° and dipping 65° to the south at a depth of 20.1 ± 0.05 km, which indicates that the earthquakes in the sequence occurred within a rupture radius of the mainshock. Detailed seismic analysis of the sequence indicates that closely located subfaults or small weak zones were related to the mainshock.

We present new 3D source fault representations for the 2019 M 6.4 and M 7.1 Ridgecrest earthquake sequence. These representations are based on relocated hypocenter catalogs expanded by template matching and focal mechanisms for M 4 and larger events. Following the approach of Riesner et al. (2017), we generate reproducible 3D fault geometries by integrating hypocenter, nodal plane, and surface rupture trace constraints. We used the southwest–northeast-striking nodal plane of the 4 July 2019 M 6.4 event to constrain the initial representation of the southern Little Lake fault (SLLF), both in terms of location and orientation. The eastern Little Lake fault (ELLF) was constrained by the 5 July 2019 M 7.1 hypocenter and nodal planes of M 4 and larger aftershocks aligned with the main trend of the fault. The approach follows a defined workflow that assigns weights to a variety of geometric constraints. These main constraints have a high weight relative to that of individual hypocenters, ensuring that small aftershocks are applied as weaker constraints. The resulting fault planes can be considered averages of the hypocentral locations respecting nodal plane orientations. For the final representation we added detailed, field-mapped rupture traces as strong constraints. The resulting fault representations are generally smooth but nonplanar and dip steeply. The SLLF and ELLF intersect at nearly right angles and cross on another. The ELLF representation is truncated at the Airport Lake fault to the north and the Garlock fault to the south, consistent with the aftershock pattern. The terminations of the SLLF representation are controlled by aftershock distribution. These new 3D fault representations are available as triangulated surface representations, and are being added to a Community Fault Model (CFM; Plesch et al., 2007, 2019; Nicholson et al., 2019) for wider use and to derived products such as a CFM trace map and viewer (Su et al., 2019).

The island of Lombok in Indonesia is located between the Indo-Australian and Eurasian subduction trenches and the Flores back-arc thrust, making it vulnerable to earthquakes. On 29 July 2018, a significant earthquake Mw 6.4 shook this region and was followed by series of major earthquakes (Mw>5.8) on 5, 9, and 19 August, which led to severe damage in the northern Lombok area. In this study, we attempt to reveal the possible cause of the sequences of the 2018 Lombok earthquakes based on aftershock monitoring data. Twenty stations were deployed to record earthquake waveform data from 4 August to 9 September 2018. In total, 3259 events were identified using 28,728 P- and 20,713 S-wave arrival times during the monitoring. The aftershock hypocenters were determined using a nonlinear approach and relocated using double-difference method. The moment magnitude (Mw) of each event was determined by fitting the displacement spectrum amplitude using a Brune-type model. The magnitudes of the aftershocks range from Mw 1.7 to 6.7. The seismicity pattern reveals three clusters located in the Flores oceanic crust, which fit well with the occurrences of the four events with Mw>6. We interpret these events as the main rupture area of the 2018 Lombok earthquake sequence. Furthermore, an aseismic zone in the vicinity of Rinjani extending toward the northwestern part of Lombok was observed. We propose that the crust in this area has elevated temperatures and is highly fractured thus inhibiting the generation of large earthquakes. The aseismic nature is therefore an artifact of the detection threshold of our network (Mw 4.6).

The present study presents the analysis of a moderate earthquake (Mw 4.0) and its largest aftershocks located along the Red Sea coast, southwestern Saudi Arabia, with the aim to understand the enigma of peculiar seismicity in the Arabian Shield. We analyzed a high-quality waveform dataset collected from ten well-recorded earthquakes of moment magnitude ranging from 2.0 to 4.0 in order to determine the hypocenter locations and focal mechanisms. The focal mechanisms were retrieved from the regional moment-tensor inversion for the mainshock and using the P-wave polarities for the corresponding aftershocks of Mw ≥ 2.0, respectively. The focal mechanism solutions were inverted to retrieve the seismogenic stress using the stress tensor inversion. The combination of the nearby fault trends and the obtained results from hypocentral relocations, focal mechanism solutions, and stress tensor inversion emphasizes that the NE fault trend is likely to be the causative fault resulting in the 2017 Namas earthquake sequence, implying that the local tectonic setting is incompatible with the large-scale regional tectonics of the Red Sea opening. On the contrary, estimates of low-stress drops exhibit typical values compatible with those reported for the shallow plate-boundary earthquakes that occurred in the Red Sea, suggesting that the existing weak zones in the southernmost part of the Arabian Shield may be attributed to the large-scale regional tectonics of the Red Sea opening.

The paper assessed the predictability of the seismic energy rate and large earthquakes in southern Europe and the Mediterranean according to the data from the United States Geological Survey catalog for 1900–2016. A nonlinear second-order differential equation is used as a mathematical model, and optimization and predictability estimation algorithms are this author’s. The estimates show a high predictability of energy rate trends. For 84 out of 217 large earthquakes in the region, we have found foreshock predictability, and aftershock predictability for 200 earthquakes. The predictability of large earthquakes begins to be seen at small (1.5–7.5 km) radii of hypocenter samples, increases rapidly at medium (15 and 30 km) radii, then more smoothly increases at a radius of 60 km and decreases slightly at a radius of 150 km. The predicted distance in time averages tens of days for foreshock predictability and thousands of days for aftershocks. The results demonstrate good prospects for the approximation–extrapolation approach to the prediction of both earthquakes themselves and the subsequent aftershock decay of seismic activity.

The M L 6.1 earthquake that occurred on June 18, 2013 in Kuzbass is the strongest seismic event related to mining operations in this region . O pinions about its genesis differ . On the one hand, its hypocenter and most aftershocks occurred directly underneath the Bachat open-pit mine, which suggests that this seismic event was due to anthropogenic impacts . On the other hand, the earthquake focus was located at a depth of several kilometers, which, according to some authors, argues against th e anthropogenic factor – the technogenic change in the parameters of the stress field was insignificant against the lithostatic pressure and, especially, the rock strength ( e.g. [Lovchikov, 2016]) . Our study aims to discover and assess an impact of the mining operations in the near-surface areas of the crust , investigate whether the Bachat earthquake was caused by the mining operations, and clarify which processes in particular were the most probable trigger s of dynamic movement in the Bachat earthquake source . The probable geometrical parameters of the fault plane were estimated from the structural and tectonic conditions of the study area and the published locations of the aftershock s [Emanov et al., 2017]. It is established that seimic events of magnitudes similar to that of the Bachat earthquake cannot be caused by the overall anthropogenic load on the area, and it is unlikely that such a strong earthquake may occur due to the direct effect s of seismic vibrations resulting from mass explosions during the mining operations . Our a nalytical models and numerical simulations , as well as the analysis of seismological observation data show that the most probable factor that initiat ed dynamic movement s in the earthquake source was the extraction of the huge rock volume and its transportation from the Bachat open-pit mine . It should be noted that the size of the zone , wherein the geomechanical initiation criteria are met , is considerably larger than the critical size of a nucleation zone for a M 6 earthquake. However , open -pit mining operations can hardly affect the localization of strong earthquake s ources. Mining operations can only trigger a seismic event that has been already prepared by the natural evolution of the crust.

We analyzed GPS data to investigate the source parameters of the 2014 North Aegean earthquake (Mw 6.9). We simultaneously inverted strike, dip, rake, average slip, moment, size, and location of the mainshock rupture using coseismic displacements. We also investigated static stress change associated with the mainshock and its influence on aftershock activity. Previous solutions of focal mechanism showed that mainshock has a nearly pure strike-slip nature. Our results also indicate that the mainshock has a right lateral strike-slip mechanism with a substantial extensional component. Average coseismic slips are 48 cm and 25 cm on the horizontal and vertical axis, respectively. Coseismic displacements reach up to 51.35 ± 4.55 mm at ~60 km from the hypocenter (LEMN station). It appears that the mainshock ruptured a fault section approximately 28 km deep and 70 km long and generated a stress drop along the ruptured segment ranging between 3 and 10 bar, with a stress increase of 3 bar at the edges of the rupture. Aftershocks were distributed to a broader area as coseismic stress increase affected a 200-km-long section of the western North Anatolian Fault.

The Hokkaido Eastern Iburi Earthquake (M = 6.7) occurred on Sep. 6, 2018 in the southern part of Central Hokkaido, Japan. Since Paleogene, this region has experienced the dextral oblique transpression between the Eurasia and North American (Okhotsk) Plates and the subsequent collision between the Northeast Japan Arc and the Kuril Arc due to the oblique subduction of the Pacific Plate. This earthquake occurred beneath the foreland fold-and-thrust belt of the Hidaka Collision zone developed by the collision process, and is characterized by its deep focal depth (~ 37 km) and complicated rupture process. The reanalyses of controlled source seismic data collected in the 1998–2000 Hokkaido Transect Project revealed the detailed structure beneath the fold-and-thrust belt, and its relationship with the aftershock activity of this earthquake. Our reflection processing using the CRS/MDRS stacking method imaged for the first time the lower crust and uppermost mantle structures of the Northeast Japan Arc underthrust beneath a thick (~ 5–10 km) sedimentary package of the fold-and-thrust belt. Based on the analysis of the refraction/wide-angle reflection data, the total thickness of this Northeast Japan Arc crust is only 16–22 km. The Moho is at depths of 26–28 km in the source region of the Hokkaido Eastern Iburi Earthquake. Our hypocenter determination using a 3D structure model shows that most of the aftershocks are distributed in a depth range of 7–45 km with steep geometry facing to the east. The seismic activity is quite low within the thick sediments of the fold–thrust belt, from which we find no indication on the relationship of this event with the shallow (

We investigate significant earthquake sequences in the observation period between 2000 and 2018 along the Tuz Golu Fault Zone in Central Anatolia. These sequences include a series of strong events with moment magnitudes (M-w) > 5.0, e.g. 30 July 2005 Bala-Ankara M-w 5.2, 20 December 2007 Bala-Ankara M-w 5J and 26 December 2007 M-w 5.6 Afar-Ankara earthquakes. The mainshocks are shallow focus strike-slip events with minor normal component at depths of 12-15 km. Focal depths of aftershocks range from 2 to 30 km. The focal mechanisms of the aftershocks are mainly normal faulting with variable strike-slip components. The geometry of focal mechanisms reveals a normal faulting regime with E-W and also NE-SW trending directions of T-axis in the entire activated region. A stress tensor inversion of focal mechanism data is performed to acquire a more accurate picture of the Tuz Gou Fault Zone stress field. The stress tensor inversion results display a predominant normal stress regime with a NNE SSW oriented maximum horizontal compressive stress (S-H). According to high-resolution hypocenter relocations of the Tuz Golu Fault Zone seismic sequences, eight clusters are revealed. The aftershock activities in the observation period between 5 March 2000 and 2 June 2018 extend from NW to SE direction. Seismic cross-sections show that a complex pattern of the hypocenter distribution with the activation of Tuz Gold Fault Zone segments. The western clusters are associated with a fault plane trending mainly N-S and dipping towards almost vertical, while the eastern clusters are related to a fault plane trending NW and dipping towards SE direction. The best constrained focal depths are mainly confined in the crust (depth < 30 km) and range from 2 to 30 km. In this study, we compare the results of an analysis of b-values from seismicity and stress tensors of focal mechanism solutions to understand their coupling in terms of faulting and earthquake hazard implications. In particular, this comparison allows to investigate the spatiotemporal correlation between b-values and stress distributions and it is therefore able to locate fault segments that have a high potential of generating large earthquakes. According to recent seismicity, b-values range from 0.5 to 1.5 along the Tuz Golu Fault Zone in Central Anatolia. The maximum horizontal compressive stress (S-H) indicates N-S and NE-SW directions while the minimum horizontal compressive stress (Sh) displays E-W. These results represent that low stress variances are correlated with high b-values. Low b-values are predominant in areas with high stress rates, consistent with a high heterogeneity of focal mechanisms.

Author(s): Konca, A Ozgun; Guvercin, Sezim Ezgi; Ozarpaci, Seda; Ozdemir, Alpay; Funning, Gareth J; Dogan, Ugur; Ergintav, Semih; Floyd, Michael; Karabulut, Hayrullah; Reilinger, Robert | Abstract: SUMMARY The 2017 July 20, Mw6.6 Bodrum–Kos earthquake occurred in the Gulf of Gokova in the SE Aegean, a region characterized by N–S extension in the backarc of the easternmost Hellenic Trench. The dip direction of the fault that ruptured during the earthquake has been a matter of controversy where both north- and south-dipping fault planes were used to model the coseismic slip in previous studies. Here, we use seismic (seismicity, main shock modelling, aftershock relocations and aftershock mechanisms using regional body and surface waves), geodetic (GPS, InSAR) and structural observations to estimate the location, and the dip direction of the fault that ruptured during the 2017 earthquake, and the relationship of this event to regional tectonics. We consider both dip directions and systematically search for the best-fitting locations for the north- and south-dipping fault planes. Comparing the best-fitting planes for both dip directions in terms of their misfit to the geodetic data, proximity to the hypocenter location and Coulomb stress changes at the aftershock locations, we conclude that the 2017 earthquake ruptured a north-dipping fault. We find that the earthquake occurred on a 20–25 km long, ∼E–W striking, 40° north-dipping, pure normal fault with slip primarily confined between 3 and 15 km depth, and the largest slip exceeding 2 m between depths of 4 and 10 km. The coseismic fault, not mapped previously, projects to the surface within the western Gulf, and partly serves both to widen the Gulf and separate Kos Island from the Bodrum Peninsula of SW Anatolia. The coseismic fault may be an extension of a mapped, north-dipping normal fault along the south side of the Gulf of Gokova. While all of the larger aftershocks are consistent with N–S extension, their spatially dispersed pattern attests to the high degree of crustal fracturing within the basin, due to rapid trenchward extension and anticlockwise rotation within the southeastern Aegean.

Throughout the years seismic hazard calculations in Peru have been developed using area sources models, having to date a great variety of models, however, since they are discretized planar models, they cannot adequately represent the continuity and subduction characteristics of the Nazca Plate. The main objective of this work is the developing of a surface subduction model (SSM), useful for seismic hazard assessments as well as the revision and control of previous models used in this sort of assessments. In this study a spatial interpolation was performed employing the Local Polynomial Interpolation method to capture short-range variation in addition to long-range trends. The data base is based on the compilation of seismic catalogs from Peruvian and international institutions such as the IGP, the USGS, the ISC and others, subsequently, in order to have independent events the elimination of duplicate events, aftershocks and foreshocks was carried out. Then, by interpolation of the focal depths of the independent events, a subduction surface model (SSM) was generated as well as a Standard Error Surface which supports a good correlation of the model. Furthermore, 14 transversal sections of the SSM was employed to compare with the hypocenter’s distributions, evidencing a good correlation with the spatial distribution of the events, in addition to adequately capturing the subduction characteristics of the Nazca Plate. Finally, a comparison was made between 2 Peruvian area models for seismic hazard and SSM developed in the present research, evidencing that seismic source models of the area type have deficiencies mainly in the depths they consider, thus is recommended the use of the present model for future seismic hazard assessments.

The source rupture process of the 2018 Hokkaido Eastern Iburi earthquake was investigated by performing a joint inversion analysis using the strong motion and geodetic data. A fault model that consists of two fault planes was constructed by considering the relocated aftershock distribution and the focal mechanisms. The inversion result showed that the large slip occurred at approximately 22 km depth, which was much shallower than the hypocentral depth. Our results showed that the rupture initiated on the minor fault plane around the hypocenter and the major fault plane started to rupture 4–6 s after the rupture initiation. Although the shape of the minor fault plane has not been clearly determined, the major fault plane appears to be high-angle, east-dipping. An additional inversion with only the major fault plane showed that the strong ground motion near the source was mainly generated from the major fault plane. The total seismic moment estimated by the inversion with the two fault-plane model was 1.1 × 1019 Nm, which yielded an M w of 6.6. Using the inversion result of the two fault-plane model, we simulated the ground surface and borehole waveforms at strong motion station IBUH03 where the large velocity pulse was observed on the ground surface. The simulations suggest that this large velocity pulse was generated from the combination of the large slip of the source and large site amplification of the velocity structure between the ground surface and borehole seismometers.

Strong earthquakes can disturb the regional stress field. The variation in the static Coulomb stress changes induced by earthquakes can explain the spatial distribution of certain aftershocks, and cascade earthquake sequences. However, the coseismic Coulomb stress change is not the only stress perturbation, interseismic and postseismic viscoelastic relaxation can also affect the evolution of the stress field. This study focuses on how stress perturbation from four strong earthquakes (the 1933 M7.5 Diexi, 1955 M7.5 Kangding, 1976 M7.2 Songpan and 2008 M8.0 Wenchuan) occurred around the Longmen Shan area in western China affect the occurrence of the 2013 Lushan M7.0 earthquake. In addition, we examine the stress evolution along the Longmen Shan fault zone and its adjacent faults to assess the regional seismic hazard. We construct a 3D viscoelastic finite element model that includes the fault structure, inhomogeneous medium, tectonic stress, gravity to simulate the evolutionary stress field when using observed Global Positioning System (GPS) vectors as the boundary conditions. The simulated results show that the coseismic Coulomb stress change induced by the Kangding, Songpan, and Wenchuan earthquakes is positive at the hypocenter of Lushan earthquake. In particular, the coseismic Coulomb stress change imparted by the Wenchuan earthquake exceeds the earthquake-triggering threshold of 0.01MPa. Additionally, the tectonic stress at the hypocenter of the Lushan earthquake increases with time, indicating the stress cumulates gradually at this location. We also compare the distribution of the earthquakes (from 1933 to 2013) with the effective and Coulomb evolutionary stress fields. Almost 70% of the earthquakes in the research region are located in the effective and Coulomb stress-enhanced areas. Our study helps to better understand the origin of Lushan earthquake and provide a theroetical basis for assesing regional seismic hazard.

This paper reports the estimation of the predictability of seismicity and large earthquakes in Kamchatka as inferred from data in the Kamchatka regional catalog for 1962–2014. The mathematical model uses a second-order nonlinear differential equation, while the optimization algorithm and the estimates of predictability are the author’s own. The estimates show a high predictability of seismicity; the extrema of prediction nonlinearity typical of large earthquakes usually occur simultaneously with similar extrema of seismicity as a whole. Overall, 220 large (K ≥ 13.3) Kamchatka earthquakes were analyzed to find that foreshock predictability was available for 200 earthquakes (~30000 determinations) and aftershock predictability for 215 earthquakes (~300000 determinations). The predictability related to large earthquakes began to be seen and was rapidly increasing at intermediate (7.5–30 km) radii of hypocenter samples. The prediction distances over time were some tens and hundreds of days for foreshock predictability and some hundreds and thousands of days for aftershock predictability. These results demonstrate very good promise for the approximation extrapolation approach to the prediction of both large earthquakes themselves and of subsequent aftershock decay of seismic activity.

A comprehensive analysis of waveforms of seismic and infrasonic vibrations from the earthquake that occurred on December 5, 2014, in the water area of Lake Hovsgol was performed. The analysis showed that the infrasonic signal recorded at the Tory station (Geophysical Observatory of the Institute of Solar-Terrestrial Physics, Russian Academy of Sciences) was formed by the sources of three generation types: local, secondary, and epicentral. The obtained results gave support to the model of epicentral infrasonic signal generation by flexural waves from an elastic ice membrane on the surface of Lake Hovsgol.

The paper discusses the results of the detection of seismic and infrasonic waves generated by a major earthquake and its aftershock (the moment magnitude MW=4.9 and MW=4.2 respectively), which occurred in northern Mongolia under Lake Hovsgool on December 5, 2014. The joint analysis of waveforms of seismic and infrasonic oscillations has shown that the signal recorded by the infrasound station of the Geophysical Observatory of the Institute of Solar-Terrestrial Physics SB RAS (ISTP SB RAS) is formed from sources of three generation types: local, secondary, and epicentral. This analysis enables us to propose a hypothesis of generation of epicentral infrasonic signal by flexural waves in an elastic ice membrane on the surface of Lake Hovsgool, which appear during the passage of seismic wave packets. This hypothesis explains the similarity between seismic and epicentral infrasonic signals, negative initial phase of epicentral infrasonic waves, and detection of a weak signal after a small-magnitude aftershock.

In this study, we investigated the main features of the causative fault of the 24 January 2020, Mw 6.8 Elazığ earthquake (Turkey) using seismological and geodetic data sets to provide new insight into the East Anatolian Fault Zone (EAFZ). We first constrained the co-seismic surface deformation and the rupture geometry of the causative fault segment using Interferometric Synthetic Aperture Radar (InSAR) interferograms (Sentinel-1A/B satellites) and teleseismic waveform inversion, respectively. Also, we determined the centroid moment tensor (CMT) solutions of focal mechanisms of the 27 aftershocks using the regional waveform inversion method. Finally, we evaluated the co-seismic slip distribution and the CMT solutions of the causative fault as well as of adjacent segments using the 27 focal solutions of the aftershocks, superimposed on the surface deformation pattern. The CMT solution of the 24 January 2020Elazığ earthquake reveals a pure strike-slip focal mechanism, consistent with the structural pattern and left-lateral motion of the EAFZ. The rupture process of the Elazığ event indicated that the rupture is started at 12 km around the hypocenter, and then propagated bilaterally along the NE-SW but mainly toward the southwest. The rupture slip has initially propagated toward the southwest (first 10 s) and northeast (4 s), and again toward the southwest (9 s). Maximum displacement is calculated as 1.3 m about 20 km southwest of the hypocenter at 6 km depth (centroid depth). The rupture stopped to down-dip around 20 km depth toward the southwest. The distribution of the slip vectors indicates that the rupture continued mostly through a normal oblique movement. Most of the moment release was released SW of the hypocenter and the rupture reached up to around 50 km. The focal mechanisms of analyzed 27 aftershocks show strike-slip, but mostly normal and normal oblique-slip faulting with an orientation of the tensional axes (NNE-SSW), indicating a normal oblique-slip, “transtensional” stress regime, parallel-subparallel to the strike of the EAFZ, consistent with SW-rupture directivity and co- seismic deformation pattern. Finally, based on the co-seismic surface deformation compatible with the distributional pattern of normal focal solutions, normal and normal oblique-slip focals of the aftershocks evidence the rupture-parallel pull-apart basin activation as a segment boundary of the left-lateral strike-slip movement of the EAFZ.

In the last decade after about 45 years of seismic quiescence, southwest Taiwan was imperiled by two strong earthquakes occurring in relatively close proximity, the 2010 MW 6.2 Jiashian earthquake and the deadly 2016 MW 6.4 Meinong earthquake. The focal mechanisms and aftershock distributions imply that both events had rupture along the NW–SE striking, shallow-dipping fault planes at the depths of 21 and 16 km, respectively, with their hypocenters horizontally separated by ~ 17 km. Here we present the Multiple SIgnal Classification (MUSIC) back projection (BP) images using high-frequency P- and sP-waves recorded by the European and Australian seismic networks, the directivity analysis using global teleseismic P waves, the distribution and predominant focal mechanisms of relocated aftershocks, and the temporal variation in the frequency-magnitude distribution (b-value) of regional seismicity to characterize the rupture characteristics of the two mainshocks and explore the potential connection between them. The results of the Meinong event indicate a unilateral, subhorizontal rupture propagating ~ 17 km northwestward and lasting for ~ 6–7 s (Jian et al. in Geophys Res Lett 44:3545–3553, 2017). The Jiashian earthquake nucleated ~ 5 km deeper and ruptured obliquely toward the NW and up-dip (~16°) direction with a shorter rupture length of ~ 10 km and a duration of ~ 4–5 s. The up-dip propagation is corroborated by the 3-D directivity analysis. Moreover, the refined aftershock locations reveal that the Jiashian sequence is confined in an elongated zone extending 15 km in the NW–SE direction and located 0–3 km shallower than the hypocenter. The Meinong aftershocks mainly form three clusters: the first one clustered around the mainshock hypocenter, the second one distributed deeper (> 20 km) and ~ 20 km northwest of the mainshock epicenter off the rupture plane, and the third one occurring farther east of the epicenter at < 10 km depth beneath the southern Central Mountain Range. As evidenced by similar magnitudes, focal mechanisms, rupture directions and propagation speeds of the two mainshocks as well as the spatial configuration of the rupture zones and aftershock distributions, we attribute their occurrence to the surface roughness and strength heterogeneity with the two relatively stronger and larger asperities on a buried oblique fault that have been recently reactivated and interacted with each other through static stress transfer, which consequently caused the ruinous destruction in SW Taiwan.

Foreshocks in the form of microseismicity are among the most powerful tools to study the physical processes that occur before main earthquakes. However, their detection and precise characterization is still sparse, especially for small-to-moderate-size earthquakes (Mw

On July 7, 2019, an earthquake occurred beneath the Molucca Sea. The BMKG said the magnitude seven quakes at a depth 36 km. The next day, aftershocks occurred continuously. This data indicates that the Maluku Subduction Zone always moves dynamically, so it needs to be aware of the potential significant earthquake that can generate a tsunami in this zone. The purpose of this study is to determine the tsunami-prone areas due to earthquake activity beneath the Molucca Sea. This earthquake relates to the dynamics of plate tectonics. The location of the earthquake source is shown by the hypocenter. This location is important to know as part of pre-disaster mitigation information so that we have the preparation to handle the tsunami threat. The study was conducted using Tsunami Numerical Simulation. Earthquake parameter using the BMKG earthquake database that validated using the GFZ earthquake database. The conclusions of this study show ten locations of tsunami-prone areas. Five locations are located in North Sulawesi Province, these are Bitung, North Minahasa, Southeast Minahasa, Minahasa, East Bolaang Mongondow, and five other locations are located in North Maluku Province, those are North Halmahera, West Halmahera, Ternate, Tidore Islands, and South Halmahera.

Significant seismicity anomalies preceded the 25 October 2018 mainshock (Mw = 6.8), NW Hellenic Arc: a transient intermediate-term (~2 yrs) swarm and a short-term (last 6 months) cluster with typical time-size-space foreshock patterns: activity increase, b-value drop, foreshocks move towards mainshock epicenter. The anomalies were identified with both a standard earthquake catalogue and a catalogue relocated with the Non-Linear Location (NLLoc) algorithm. Teleseismic P-waveforms inversion showed oblique-slip rupture with strike 10&deg, dip 24&deg, length ~70 km, faulting depth ~24 km, velocity 3.2 km/s, duration 18 s, slip 1.8 m within the asperity, seismic moment 2.0 &times, 1026 dyne*cm. The two largest imminent foreshocks (Mw = 4.1, Mw = 4.8) occurred very close to the mainshock hypocenter. The asperity bounded up-dip by the foreshocks area and at the north by the foreshocks/swarm area. The accelerated foreshocks very likely promoted slip accumulation contributing to unlocking the asperity and breaking with the mainshock. The rupture initially propagated northwards, but after 6 s stopped at the north bound and turned southwards. Most early aftershocks concentrated in the foreshocks/swarm area. This distribution was controlled not only by stress transfer from the mainshock but also by pre-existing stress. In the frame of a program for regular monitoring and near real-time identification of seismicity anomalies, foreshock patterns would be detectable at least three months prior the mainshock, thus demonstrating the significant predictive value of foreshocks.

Gorgun, Ethem/0000-0002-4563-3296 WOS:000544243200004 Ayvacik district of canakkale province was hit by four moderate size earthquakes on 6 February 2017 03:51 M-w 5.2, 6 February 2017 10:58 M-w 5.1, 7 February 2017 02:24 M-W 5.2, 12 February 2017 13:48 M-w 5.3. Centroid moment tensor solutions for 60 events with moment magnitudes (M-w) between 3.4 and 5.3 are computed by applying a waveform inversion method on data from the Turkish and Greek broadband seismic networks. The time span of data covers the period between 1 January and 28 February 2017. The mainshock is a shallow focus normal event with strike-slip component at a depth of 10 km. Focal depths of aftershocks range from 3 to 24 km. The seismic moment (M-o) of the mainshock is estimated as 7.62 x 10(16) Nm. The calculated rupture duration of the Ayvacik mainshock is 3 s. The focal mechanisms of the aftershocks are mainly normal faulting with a minor strike-slip component. The geometry of focal mechanisms reveals a normal faulting regime with NE-SW trending direction of T-axis in the entire activated region. We perform stress tensor inversion to acquire more accurate picture for the regional stress field. The stress tensor inversion results indicate a predominant normal stress regime with a WNW-ESE oriented maximum horizontal compressive stress (S-H). With respect the newly determined focal mechanisms, the effect of the propagation of the North Anatolian Fault into Aegean Sea is very clearly pronounced. According to the double-difference relocation algorithm for the 2017 Ayvacik seismic sequence, ten clusters are revealed. In order to find these clusters, we use HypoDD software. According to the relative relocations from HypoDD, we define 10 clusters along the five profiles. The aftershock activity in the observation period between 1 January and 28 February 2017 extends from W to E direction. Seismic cross-sections indicate that a complex pattern of the hypocenter distribution with the ruptured segments. The western cluster is associated with a fault plane trending mainly N-S and dipping vertical, while the eastern is related to a fault plane trending NE-SW and dipping towards NW. The best constrained focal depths operate in the approximate depth range from 3 to 24 km. Coulomb stress analysis is performed to calculate the cumulative effect of all the earthquakes of the swarm and correlate it-with the activated region. Positive lobes with stress more than 3 bars are obtained, these values are large enough to trigger failure towards NW-SE of the swarm activity.

Earthquake occurrence in the stress shadow provides a unique opportunity for extracting information about the physical processes behind earthquakes because it highlights processes other than the ambient stress change in earthquake generation. In this study, we examined the fault structure and the spatiotemporal distribution of the aftershocks of the 2019 M6.7 Yamagata-Oki earthquake, which occurred in the stress shadow of the 2011 M9.0 Tohoku-Oki earthquake, to better understand the earthquake generation mechanism. The detection and hypocenter relocation of the aftershocks led to the delineation of three planar earthquake structures consitent with their focal mechanisms. The results suggest that individual aftershocks were caused by a slip on these macroscopic planar structures. Aftershock hypocenters rapidly migrated upward from the deeper part of the major plane (fault), similar to the recent earthquake swarm sequences following the Tohoku-Oki earthquake in the upper plate. Moreover, we investigated the temporal evolution of the surface strain rate distribution in the source region using GNSS data. The east–west contraction strain rate in the source region of the Yamagata-Oki earthquake, with an E–W compressional reverse fault mechanism, changed to an E–W extension as a result of the Tohoku-Oki earthquake, and it continued until the occurrence of the Yamagata-Oki earthquake. The upward hypocenter migrations, together with the earthquake occurrence in the stress shadow and in the E–W extension strain rate field, suggest that a reduction in the fault strength due to uprising fluids contributed to the occurrence of this earthquake sequence. Localized aseismic deformations, such as aseismic creeps, beneath the fault zone may also have contributed to the earthquake occurrence. The results support the hypothesis that aseismic processes in the deeper part of the fault play crucial roles in the occurrence of shallow intraplate earthquakes.

Despite our general knowledge of earthquake processes, it is still not fully understood how earthquake ruptures nucleate and propagate and why they stop. Also, the controlling factors of the frequency and of the size of earthquakes are subject of ongoing research. We aim to address these questions with a comprehensive study of seismicity in deep South African gold mines. We find here the unique situation that the seismicity consists of both induced earthquakes and aftershocks triggered by the M5.5 Orkney earthquake which occurred in August 2014. We separate the cataloged seismicity and group the events into three classes: the aftershock sequence, seismicity induced by fluids, and seismicity induced by mining activities. We examine statistical properties of earthquakes in each of the three classes. We conclude that the magnitude statistics of both aftershocks and induced earthquakes are influenced by the finite size and geometry of the rock volume of stress perturbation resulting in an absence of larger magnitude events. The magnitude frequency distributions obey the Lower Bound model of magnitude probability. The statistics of dynamic stress drop of aftershocks and induced earthquakes satisfy log-normal distributions but the value range is different, it means that aftershocks are generally characterized by higher stress drops. Another key aspect in our study is the imaging of the propagating rupture of the M5.5 earthquake. We apply the back projection imaging approach using seismological data from two different local networks and retrieve similar results. We conclude that the rupture of the M5.5 earthquake propagated predominantly unilaterally, nearly from North to South over a distance of about 6km. The hypocenters of the aftershock sequence are situated unilaterally in respect to the hypocenter of the main shock and are aligned to the South which confirms the obtained rupture propagation image and the directivity of the main shock.

In this study, we investigate the source rupture process of the 2018 Hokkaido Eastern Iburi earthquake in Japan (MJMA6.7) and how the ground motion can be reproduced using available source and velocity models. First, we conduct a multiple-time-window kinematic waveform inversion using strong motion waveforms, which indicates that a large slip area located at a depth of 25–30 km in the up-dip direction from the hypocenter was caused by a rupture propagating upward 6–12 s after its initiation. Moreover, the high-seismicity area of aftershocks did not overlap with the large-slip area. Subsequently, using the obtained source model and a three-dimensional velocity structure model, we conduct a forward long-period (

Summary The largest recordable earthquake in Thailand occurred on Chiang Rai province at the conjugate of Phayao fault zone between Mae Lao and Phan strike-slip faults. To explain the source processing of the earthquake mainshock, the modern teleseismic waveform inversion method was applied to investigate the fault rupture propagation along the assumed model fault plane. According to our results, the estimated seismic moment is 0.36 x 1019 Nm (Mw6.3) with around 9 second rupture-time duration. The largest potency-slip was around 1 m being located on the middle of Mae Loa aftershock zone. The rupture propagation originated from the hypocenter and moved toward south direction. Then, the rupture mechanism initiated on Mae Lao fault and sequentially migrated to southwestward. Our rupture process was supported by the first aftershock occurring on Mae Lao fault trend. Furthermore, our potency rate-density demonstrated low slip-rate on the transition zone related to the low seismicity gap of the relocated aftershocks.

Since most continental earthquakes occur in the brittle layer instead of ductile layers, the focal depth of an earthquake provides an important constraint on the rheology structure of fault zone. According to the results of previous studies, the typical depth of brittle-ductile transition zone on mature faults is about 10 km. However, several moderate earthquakes occurred on young faults in stable cratons, and focal depths of those events are very shallow (e.g. the 1993 M w6.1 India Killari earthquake and a series of shallow earthquakes in Australia). Those observations suggest that depth of the brittle- ductile zone on young faults in cratons is shallower than that on mature faults. In this study, we report a case on a young fault in a tectonically active region that has not been well studied and discuss implications for the rheology structure. On October 7, 2014, an M w6.1 earthquake occurred in Jinggu, Yunnan (China), the southeastern part of the tectonically active Yunnan-Myanmar block, followed by two aftershocks of M >5 in December. According to historical seismicity and geologic studies in this region, it is suggested that the seimogenic fault of the Jinggu earthquake is a young fault. We use data from global and regional seismic networks, as well as aftershock data from the Lozhadu reservoir network and temporary stations, to study focal depths of the mainshock and aftershocks. The hypocenter of a reference event ( M w4.3) that was recorded with two close temporary stations is determined, and then the hypocenters of the mainshock and two moderate aftershocks are relocated with a relative method based on Pn/Pg arrival times. The relocated hypocenter of the mainshock is 9.5 km deep, whereas the hypocenters of the two aftershocks are about 10 km deep. The centroid depth is investigated with the CAP method based on waveform modeling. Both teleseismic and regional waveforms are used to invert for the centroid depth of the mainshock. The optimal waveform fit is obtained with a depth of 5 km. The two aftershocks are studied with regional waveforms only and the results are close to the hypocenter depths. Since the hypocenter indicates the initial point of rupture whereas the centroid location is close to the center of the main rupture patch, we propose the mainshock initiated at 10 km depth and expanded to the brittle layer at shallower depth. For two aftershocks, the consistency of hypocenter depth and centroid depth suggests circular rupture patterns. Therefore, we propose the bottom of the seismogenic layer and brittle-ductile transition zone is about 10 km deep. This hypothesis is also supported by other observations. Magnetotelluric study in this region reveals a low conductivity layer above 10 km that is regarded as brittle rock, whereas the high conductivity layer beneath 15 km is regarded as ductile rock. Furthermore, a regional rheology model, which is obtained with surface temperature, seismic velocity, and GPS data, also suggests the brittle-ductile transition zone is at about 9 km depth. In summary, the depth of the brittle-ductile transition zone on the young fault in the tectonic active region is similar to that of mature faults, which is useful information for lithosphere geodynamics studies.

Southeast of Iran experienced eight destructive earthquakes during 30 years from 1981 to 2011. Six of these events with M > 6.5 were fatal and caused great human and financial losses in the region. The 1981 July 28 (Mw 7.2) Sirch earthquake with 65 km surface rupture was the largest event in this region since 1877 and with other three earthquakes occurred in Golbaf-Sirch region during 17 years. The 26 December 2003 (Mw 6.6) Bam earthquake was one of the most destructive events in the recorded history of Iran. There were more than 26,000 killed, 30,000 to 50,000 injured people, and more than 100,000 were homeless. We calculated the static coulomb stress changes due to this earthquake sequence (four earthquakes) between 1981 and 1998 on the Golbaf-Sirch right-lateral fault and the Shahdad reverse fault and a slow slip on the Shahdad fault. Our calculations showed positive stress changes due to previous events on the ruptured plane of next earthquake. For example, the rupture plane of the 14 March 1998 (Mw 6.6) Fandoqa earthquake received a maximum positive stress change about 2.3 MPa. Also, some parts of the surrounding faults received positive stress changes due to these events. Stress changes on the planes of other four events until 2011 were calculated in this study. The 26 December 2003 (Mw 6.6) Bam earthquake and the 20 December 2010 (Mw 6.5) first Rigan earthquake received negligible (about thousandth (0.001)) negative stress changes in this sequence. The last event in our study area, the 27 January 2011 (Mw 6.2) second Rigan earthquake, experienced more than 0.5 MPa coseismic coulomb stress changes especially in its hypocenter and according to our calculations, it is mostly due to the first Rigan event. By using well-located aftershocks of the Rigan earthquake, we investigated the correlation between coulomb stress changes and aftershocks distribution. Calculated coulomb stress changes due to these two events on the optimally oriented strike-slip faults for the first event showed that most of the well-located seismicity occurred in regions of stress increase and majority of them concentrated near the ruptured plane where the stress changes are in the highest value. Based on our computation for the second event, it would be concluded that most of the aftershocks located in the places that imposed stress are positive and some of them are in places where the imposed stress changes are zero or very small. So, there is a good correlation between coulomb stress changes and aftershocks distribution for both Rigan events. Calculating imparted coulomb stress changes that resolved on the nodal planes of the Rigan first event aftershocks has also been considered to examine whether they were brought closer to failure or not by using different fault friction. Various values of effective coefficient of friction (0.2, 0.4, and 0.8) were used to find the best value of fault friction that produces the highest gain in positively stressed aftershocks. Based on these calculations, majority of aftershocks received positive stress changes by increasing the effective coefficient of friction.

The 2008 M_w 5.2 Mt. Carmel earthquake is the largest earthquake in the last 50 yrs in southeastern Illinois, near the north termination of the north‐northeast‐trending Wabash Valley fault system (WVFS). The earthquake shows almost pure strike‐slip focal mechanism, but it is still uncertain which nodal plane (NP) is the ruptured fault plane. To resolve the fault plane, we determine rupture directivity of the earthquake via the relative centroid method. We begin with inverting the point‐source solution (strike 297°/dip 84°/rake 1° for NP1, strike 206°/dip 89°/rake 173° for NP2, and centroid depth 16 km) and then determine the relative location between the centroid and hypocenter via regional waveform fitting. Two M 4+ aftershocks are used as reference events, and the waveform time shifts of reference events with respect to the 1D velocity model are used to calibrate the path effects. The results show that the Illinois mainshock ruptured to east‐southeast along the 297° NP for about 2–3 km, consistent with relocated aftershock distribution, and we infer that the sinistral causative fault connects the north‐northwest‐trending La Salle anticlinal belt and the north‐northeast‐trending WVFS.

Seismotectonic features of the Ahar-Varzaghan complex fault system are reviewed by the mainshocks/aftershocks study of the 2012 Ahar-Varzaghan earthquake doublet (Mw = 6.5 & Mw = 6.3). The early aftershocks were accurately monitored by a dense local seismological network. The mainshocks hypocenters were relocated using a new velocity model for the area. The regional displacement waveforms of the mainshocks and large aftershocks were jointly inverted for their moment tensors and centroids. Precisely located aftershocks are mostly distributed in three distinct clusters. These E-W trending clusters are situated to the north of the surface rupture over a ∼30 km long and 10 km wide zone, extending from the ground surface down to the depth of ∼15 km. Based on our results, the first mainshock (M1) with Mw = 6.5 nucleated 3 km east of the surface rupture at a depth of 10 km. The rupture evolved toward the west and shallower depths on an almost E-W striking right-lateral strike-slip vertical fault plane and produced ∼12 km surface rupture. Its main energy was released at about 5 km west of the hypocenter at an average depth of 5 km. About 11 min later, the second mainshock (M2) with Mw = 6.3 nucleated 5 km northwest of the M1 hypocenter and at a depth of 15 km. It occurred on an ENE-WSW striking, north-dipping (65° to 70°) fault plane with a dominant reverse mechanism and strike-slip component. The M2 related rupture also expanded to the west and to shallower depths and released most of its energy ∼5 km west of the respective hypocenter at an average depth of 11 km. The eastern aftershocks mainly show the right-lateral strike-slip mechanism on almost E-W trending fault, close to the location of the first main centroid (M1). The northern aftershocks mostly include reverse mechanisms with strike-slip component near the M2 centroid. A big gap of aftershock activity is observed close to the M1 centroid location, most likely associated with the area of maximum slip. We do not observe any N-S lineament of aftershocks neither near the M2 hypocenter and nor its centroid, thus making the suggestion of some previous investigators that M2 ruptured an N-S trending fault unlikely.

Modelling of earthquake source locations and parameters infers seismogenesis of earthquakes. In this study, we modelled the earthquake source locations through hypocenter location algorithm using the difference in arrival time of P and S waves and source parameters through the Levenberg–Marquardt non-linear inversion method using S-wave spectra. A total of 340 aftershocks of 2001 Bhuj mainshock ( $$ 1.8 \le M_{w} < 4.3 $$ ), which have occurred in Kachchh, Gujarat, India from January 2014 to January 2015, are located in this study. Out of 340 aftershocks, digital waveforms of 78 aftershocks ( $$ 2.2 \le M_{w} < 3.9 $$ ) are used for estimation of the earthquake source parameters. The results obtained from earthquake locations show two clusters of seismicity along the Kachchh Mainland Fault (KMF) and North Wagad Fault (NWF) and three felt events ( $$ M_{w} \ge 3.0 $$ ); one along the Katrol Hill Fault (KHF) ( $$ M_{w} = 3.3 $$ ), two along the Banni Fault (BF) ( $$ M_{w} = 3.0,3.2 $$ ). The generation of these three felt events is attributed to the triggering mechanisms caused by the migration of fluids or the stress pulse generated by the 20 MPa stress drop of the Mw 7.7 Bhuj earthquake. A marked concentration of events is noticed in 15–30 km depth range, which could be attributed to the presence of a mafic intrusive body, resulting in stress build-up for earthquake generation in this region. The results of source parameters; seismic moment (M0), source radius (r) and stress drop (Δσ) vary from 1.86 × 1012 to 3.2 × 1015 N m, 146–262 m and 0.04–5.73 MPa, respectively. The maximum stress drop value is estimated to be 5.73 MPa at 24 km depth for the largest studied event of $$ M_{w} $$ 3.9. Large stress drops are confined to the 8–33 km depth range, which indicates the probable existence of the base of the seismogenic layer in this depth range. This observed large stress drops could be attributed to stresses induced by crustal mafic intrusive bodies and the presence of aqueous fluids in the lower crust below the region.

The 12 February 2018 M W4.3 Yongqing earthquake, is the largest shock around the Beijing area since the 2006 Wenan M S5.1 earthquake. Similar to the 2006 Wenan earthquake and many other earthquakes in this region, very few aftershocks were recorded after the Yongqing mainshock. It raises a question whether the isolated earthquake sequence is related or not to the source characteristics. To answer this question, investigations of the source mechanism and rupture process are necessary. We estimated the moment tensor solution and rupture process by inverting seismograms of 38 local broadband stations within the epicentral distances of 200 km. The duration and shape of the source time function were considered in the moment tensor inversion. Through a grid search, we found a centroid depth of about 16 km of the earthquake. The scalar seismic moment is 3.1×1015 Nm, equivalent to amoment magnitude M W4.3. From the source time functions, the rupture duration is about 3.5−4.0 s. The mechanism solution suggested a strike slip event, with a minor normal-faulting component. Combined with the geological background, the NE striking and SE dipping nodal plane was identified as the causative fault. The rupture model was estimated on the NE striking fault plane by using a linear technique. From the static slip distribution, the peak slip is about 1.3 cm, and the rupture dimension is approximately 2 km. Most slips are concentrated around the major slip asperity. The source time function suggests a source duration of about 3.5 s, and the peak moment rate appearing at 1 s, which are well consistent with those from the moment tensor inversion. The earthquake ruptures expanded from the hypocenter in a circular dislocation model at 0−2 s. A few weak slips appeared at shallow depth after 2 s, which might be caused by errors in observation and Greens functions and were less reliable. Both the peak slip and the rupture dimension of the 2018 Yongxing earthquake are higher than the values of an M W4.3 earthquake estimated from existing earthquake scaling law. They may provide additional constraints for further correction of the parameter-magnitude scaling laws. The maximum and average stress drops are about 0.6 and 0.3 MPa, respectively, remarkably lower than 10 MPa of a typical intraplate earthquake. The relatively low stress drop indicates that the fault strength is weak. Seismic tomography in the Huabei area has found that the crust is fractured, which may lead to weak faults there. The fractured crust and weak fault can block the stress transformation to some extent, and are likely to cause isolated earthquake sequence.

Since the occurrence of the 2008 Wenchuan earthquake ( M s8.0), many researchers have conducted extensive seismological and geophysical observations and investigations and obtained important results about the Longmenshan fault zone. Crustal structure inferred from local tomography shows that seismic velocity exhibits significant changes across the Wenchuan earthquake hypocenter from the south to the north. To the south,obvious low-velocity (low-V) anomalies exist, whereas strong lateral heterogeneities are revealed to the north, which may explain why the aftershocks extend northeastward. The Wenchuan earthquake occurred at the boundary between high-velocity (high-V) and low-V anomalies and a significant low-V zone is revealed below the mainshock hypocenter, suggesting that the nucleation of the Wenchuan earthquake was related to partial melts and/or fluid effects and associated with the reduction of effective normal stress on the fault plane, due to hightemperature and highpressure in the Longmenshan fault zone caused by the India-Asia collision. The upper-mantle structure inferred from teleseismic tomography shows that the Longmenshan fault zone is located in the transition zone from low-V anomalies beneath the Songpan-Ganzi block to high-V anomalies beneath the Sichuan basin. This structural feature extends down to 200−300 km depths. High-V anomalies in the mantle transition zone are connected with those in the upper mantle beneath the Burma arc, indicating that the Wenchuan earthquake could be associated with upwelling of hot and wet materials in the big mantle wedge formed by the deep subduction of the Indian plate. These results suggest that the generation of the Wenchuan earthquake was related to structural heterogeneities in not only the crust but also the upper mantle. In addition,high-poisson’s ratio and high-conductivity anomalies are revealed beneath the Wenchuan earthquake source area, which may also reflect lower crustal flow that is compressed and moving eastward. When the eastward flow encountered the strong Sichuan basin block around the Longmenshan fault zone, the flow is further compressed upward along the margin of the Sichuan basin, which could have caused the fault ruptureand generated the Wenchuan earthquake. Some geologists found a vertical co-seismic displacement of 6.5 m in the Longmenshan fault zone, suggesting that there exists an imbricate structure due to the crustal shortening. Such a shortening mechanism could rupture the seismogenic fault leading to the large earthquake. The Zipingpu reservoir is located very close to the Wenchuan earthquake epicenter. It is still debated whether the reservoir triggered the 2008 Wenchuan earthquake or not. Although some simulation results suggest that the Wenchuan earthquake could be triggered by the reservoir, there are large differences in the estimated stress in the hypocentral area due to large uncertainties of the physical parameters adopted in the numerical simulations. Hence, the physical parameters should be determined precisely in future investigations so as to clarify the effect of the reservoir on the seismogenesis. We also discuss the causal mechanism of the 2013 Lushan earthquake ( M s7.0) and future seismic risk in the gap between the aftershock zones of the Wenchuan and Lushan earthquakes.

The large M7.3 aftershock occurred 17 days after the 2015 M7.8 Gorkha earthquake. We investigate if this sequence is mechanically favored by the mainshock via time-dependent fluid migration and pore pressure recovery. This study uses finite element models of fully-coupled poroelastic coseismic and postseismic behavior to simulate the evolving stress and pore-pressure fields. Using simulations of a reasonable permeability, the hypocenter was destabilized by an additional 0.15 MPa of Coulomb failure stress change (∆CFS) and 0.17 MPa of pore pressure (∆p), the latter of which induced lateral and upward diffusive fluid flow (up to 2.76 mm/day) in the aftershock region. The M7.3 location is predicted next to a local maximum of ∆p and a zone of positive ∆CFS northeast of Kathmandu. About 60% of the aftershocks occurred within zones having either ∆p > 0 or ∆CFS > 0. Particularly in the eastern flank of the epicentral area, ~83% of the aftershocks experienced postseismic fluid pressurization and ~88% of them broke out with positive pore pressure, which are discernibly more than those with positive ∆CFS (71%). The transient scalar field of fluid pressurization provides a good proxy to predict aftershock-prone areas in space and time, because it does not require extraction of an assumed vector field from transient stress tensor fields as is the case for ∆CFS calculations. A bulk permeability of 8.32 × 10−18 m2 is resolved to match the transient response and the timing of the M7.3 rupture which occurred at the peak of the ∆CFS time-series. This estimate is consistent with the existing power-law permeability-versus-depth models, suggesting an intermediately-fractured upper crust coherent with the local geology of the central Himalayas. The contribution of poroelastic triggering is verified against different poroelastic moduli and surface flow-pressure boundaries, suggesting that a poroelastic component is essential to account for the time interval separating the mainshock and the M7.3 aftershock.

We measured the location of the mainshock of the November 18, 2017, earthquake ( M 6.9) that occurred in Mainling County, Nyingchi City, Tibet, by using arrival-time data recorded by temporary seismic stations deployed on the Namche Barwa syntaxis and surrounding regions. The seismic stations offered good coverage of the hypocenter region and provided a good opportunity to precisely locate the mainshock and aftershock sequences. The Mainling earthquake was located at 29.87° N, 95.05° E, and the focal depth is 12 km. We investigated the focal mechanism of the mainshock by applying the W phase source inversion algorithm from the broad-band records of global seismic networks accessed from the IRIS data center. First, we resampled the original waveform to 1 SPS and then removed the mean, trend, and instrument response of the waveform, filtering it with a bandwidth of 150–70 s. After inversion, we obtained a solution with M w = 6.45 and a best double couple given by strike/dip/rake=319.9°/61.7°/101.0°. This result suggested that the seismogenic fault of the Mainling earthquake was characterized by a high-angle and NE-dipping thrust. We determined the location of the aftershock sequences by applying Geiger and double difference (DD) algorithms. Due to the lack of the local-scale tomographic model and reliable Poisson’s ratio data in the hypocentral region, we used only the P-wave arrival times to locate the aftershock sequences. We determined absolute locations for 148 aftershocks with high quality P-wave arrivals recorded by 14 temporary seismic stations and estimated the locations using the 1-D velocity model, in which the P-wave velocity was 6 km/s in the upper crust, 6.6 km/s in the lower crust, and 8.1 km/s in the uppermost mantle. The location errors were 0.6 km in the horizontal direction with an average residual of 0.07 s. In addition, we used the DD location algorithm in an attempt to minimize uncertainties in the aftershock locations. It was confirmed that the DD location algorithm greatly improved the relative location accuracy of events that are close to one another compared to the traditional Geiger location method. After DD location, the location errors of the aftershock sequences are 0.23, 0.26, and 1.43 km in the north-south, east-west, and vertical directions, respectively. The results indicated that the aftershocks were distributed along both sides (NW and SE directions) of the mainshock, which spread approximately 40 km in length and 10 km in width. Results of the focal depth profiles suggested that the aftershocks were located at depths above 15 and 10 km in the NW and SE directions of the mainshock, respectively. The aftershocks were mainly located at two sub-parallel faults. In the first two hours, the M 6.9 Mainling earthquake and aftershocks were mainly distributed at fault F1, and after that the aftershocks were distributed at fault F2. This suggested that the Xixingla fault zone is composed of a series of high-angle and NE-dipping faults, and the M 6.9 Mainling earthquake occurred at the south most one. The M 6.9 Mainling earthquake triggered seismic activities of a sub-parallel fault located to the northeast. The occurrence of the M 6.9 Mainling earthquake indicates that the Namche Barwa syntaxis is still in a high tectonic compression state at present. The Namche Barwa terrane continues moving towards the NE direction, leading the shortening deformation in the interior of the terrane. It is possible that a large earthquake may occur at Motuo and Mainling faults in the future along both sides of the Namche Barwa syntaxis.

Yogyakarta earthquake, Mw 6.3, 27 May 2006 had killed 5,571 victims and destroyed more than 1 million buildings. This incident became the most destructive earthquake disaster over the last 11 years in Indonesia. Earthquake mitigation plan in the area has been carried out by understands the location of the fault. The location of the fault is still unclear among geoscientists until now. In this case, analysis of the aftershocks using oct-tree importance sampling method was applied to support the location of the fault that responsible for the 2006 Yogyakarta earthquake. Oct-tree importance sampling is a method that is recursively subdividing the solution domain into exactly eight children for estimating properties of a particular distribution. The final result of the subdividing process is a cell that has a maximum Probability Density Function (PDF) and identified as the location of the hypocenter. Input data consists of the arrival time of the P wave and S wave of the aftershocks catalog from 3-7 June 2006 and the coordinate of the 12 seismometers, and 1D velocity model of the study area. Based on the hypocenter distribution of the aftershocks data with the proposed method show a clearer trend of the fault compared with the aftershocks distribution calculated with theHypo71program. The fault trend has a strike orientation of N 42° E with a dip angle of 80° parallel with the fault scarp along the Opak River at the distance of about 15 km to the east. This fault trend is similar with the fault orientation obtained using the Double Difference Algorithm.

The paper addresses the collection and analysis of new data on aftershocks that occurred within 20 days of the main shock of the December 7, 1988, Spitak earthquake, Mw = 6.8. The data were used to improve the location of aftershock hypocenters and magnitudes. Available data concerning this 20-day period were the least reliable in terms of completeness, representativeness, and the accuracy of hypocenter location and, in particular, estimation of energy classes and magnitudes. New data were retrieved from the records and bulletins of the seismic stations of the regional and global networks. Hypocenter parameters were determined by means of the minimization of wave travel-time residuals and subsequent double-difference hypocenter relocation. Digital records of the Obninsk and Arti seismic stations (Δ = 15°–18°) and five more distant stations (Δ = 34°–53°) were used to more accurately estimate the surface-wave magnitude of the main shock and strongest aftershock. The aftershock catalog of the Spitak earthquake was substantially revised. First, the previous hypocenter locations (Aref’ev et al., 1991) were improved using the double-difference method; second, new data were retrieved from the bulletins of Caucasian seismic stations. The minimum magnitude of completeness (M c = 1.9) of the new catalog for the first 20 days after the main shock (when there were no epicentral observations) is the same as that for the period from December 7, 1988, to December 31, 1989. The new catalog contains information on 2090 aftershocks with magnitude M = 1.9 and more for the period from December 7, 1988, to December 31, 1989. The double-difference method allowed the location of the epicenters of clustered earthquakes to be reliably estimated with a longitude error of no more than 4.6 km, a latitude error of 4 km, and a depth error of 5 km. The new spatial distribution of the aftershock hypocenters is better correlated with the tectonic setting than the old data. The new catalog can be used to assess seismic hazard after strong earthquakes in the region.

The M s7.0 Jiuzhaigou earthquake with the hypocenter depth of 20 km occurred at the NNW extension line of the Huya fault located next to the eastern boundary of the Bayan Har block. The focal mechanism solution of the mainshock based on the W-phase source inversion showed a sinistral slip and the moment magnitude was M w 6.5. Over 3000 aftershocks were recorded within six days, among which the largest was M 4.8 and three others were larger than M 4.0. The value of the aftershock frequency attenuation coefficient P was 1.03, and the b value of the magnitude-frequency relationship was 0.68. The aftershock activities were relatively inactive but the attenuation was normal. Relocation results of 603 aftershocks were obtained by using the double-difference earthquake location algorithm and the location errors of the EW, NS and the vertical directions were 0.81, 0.84 and 1.59 km respectively. The mainshock occurred in the center of the aftershock region and ruptured on both sides along the strike direction. The aftershocks were distributed linearly along a NNW direction, and the fault plane was nearly vertical with a length over 30 km and a width of about 15 km. Aftershocks were more concentrated in the southeast than in the northwest. The focal mechanism solutions of most aftershocks in the central and southeastern regions were consistent with the mainshock, while those in the northwest were thrust-types, which might be related to the Minjiang fault. Meanwhile, the regional stress was released more in the central and southeastern regions than in the northwest. The maximum intensity investigated in the damage zone was up to IX. The maximum intensity region was located in scenic areas, therefore few people were present when the earthquake occurred at night. The peak acceleration was highest in the NS records, followed by the EW and vertical records. The design spectra of intensity for Jiuzhaigou was VIII, which is higher than the acceleration response spectra obtained from the Jiuzhaibaihe station, the nearest station which had strong motion recordings. These aforementioned factors may have contributed to the minor casualties resulting from this earthquake. The aftershock regions of the Jiuzhaigou earthquake, the 1973 M s6.5 Songpan northeast earthquake, and the 1976 M s7.2 Songpan earthquake are connected along the Huya fault which may extend to the northwest in the deep earth. Considering the rupture displacement distribution of the mainshock, the focal mechanism of aftershocks, and the stress drop, strong aftershocks are unlikely to occur in the central and southeastern Jiuzhaigou aftershock regions; however, the development of seismicity in the northwest and the Minjiang River fault should be noted. Static Coulomb stress triggering of other 4 earthquakes larger than M 6.5 occurred near the Ganqingchuan border were calculated using the Coulomb 3.3 software. The results indicated that the Jiuzhaigou earthquake was triggered by the Wenchuan earthquake and related to the stress adjustment in the study area.

We report evidence for frictional afterslip at shallow depths (about 5 to 7 km) during a small‐magnitude seismic sequence (with ML